Gas analyzers of the non-dispersive type typically operate on the premise that the concentration of a designated gas can be measured: (1) by passing a beam of infrared radiation through the gas, and (2) ascertaining the attenuation of the energy in a narrow wavelength band absorbable by the designated gas with a detector capable of generating an electrical output signal proportioned to the energy in the band passing through the gas. Examples of such analyzers are disclosed in U.S. Pat. Nos. 4,859,858, 4,859,859, 4,914,720, and 5,092,342.
Medical applications of these gas analyzers include the monitoring of end-tidal carbon dioxide, i.e., the concentration of carbon dioxide in a patient's exhalations. This expired carbon dioxide level can be employed by medical personnel to monitor the operation of a mechanical ventilator hooked up to the patient to assist him or her in breathing. In one typical arrangement, a system consisting of a nasal cannula through which the patient exhales can be employed so that expired breath is transmitted through a line to the gas analyzer. An example of a prior art device employing a nasal cannula is shown and described in U.S. Pat. No. 4,958,075.
In a typical medical application of a gas measuring apparatus for measurement of blood gases, a replaceable or interchangeable sample cell or cuvette is employed to connect a tube inserted into the patient's trachea to the plumbing of a mechanical ventilator. The sample cell confines the expired gases to a flow path with a precise, transverse dimension, and it furnishes an optical path between an infrared radiation emitter and an infrared radiation detector unit, both components of a gas analyzer "bench".
The infrared radiation traverses the gases in the sample cell adaptor where it is attenuated because part of the radiation is absorbed by the designated gas being analyzed. The attenuated beam of infrared radiation is then filtered to eliminate energy of frequencies lying outside the narrow band absorbed by the gas being measured. The infrared radiation in that band impinges upon a detector which consequently generates an electrical signal proportional in magnitude to the intensity of the infrared radiation impinging upon it.
The prior art systems disclosed in U.S. Pat. Nos. 4,859,858, 4,859,859, 4,914,720, 4,958,075, and 5,092,342 each include a gas analyzer and a disposable sample cell or cuvette. The sample cell is designed to be inserted into the airway of a patient on a ventilator and includes a pair of opposed windows having a line of sight positioned transverse to the sample cell so as to allow a beam of infrared radiation to pass therethrough. The sample cell is designed to be placed into the sensor unit so that the windows are in alignment with the beam path of the infrared source described above.
The gas analyzer systems disclosed in U.S. Pat. Nos. 4,859,858, 4,859,859, and 4,914,720 measure the carbon dioxide content in a respiratory gas. The respiratory gas flows through a sample cell through which an infrared beam is directed. The radiation originates from a pulsed infrared radiation source and is directed by a parabolic mirror. The mirror collimates the radiation emitted into a beam of parallel rays and focuses this beam along an optical path through the sample cell and directly onto an infrared sensitive detector located downstream of the cell. The beam is attenuated as it transverses the gas mixture because part of the radiation is absorbed by the CO.sub.2 gas. The detector measures the amount of attenuation caused to the infrared beam by the respiratory gas. The amount of attenuation corresponds to the concentration of CO.sub.2 in the respiratory gas.
To improve the sensitivity of the detector disclosed in the '859 patent, an optical filter is incorporated in front of the detector to pass a narrow band of only those wavelengths of infrared radiation absorbed by the CO.sub.2. A wavelength of approximately 4.3 microns is conventionally selected for this purpose. The remaining infrared radiation in the band impinges upon the detector. The detector then generates an electrical signal proportional in magnitude to the intensity of the infrared radiation impinging upon it, i.e., the concentration of CO.sub.2.
To increase the accuracy in measuring CO.sub.2 concentration, a second detector-filter pair is employed which is positioned juxtaposed or adjacent to the first detector-filter pair. The second filter is designed to pass a similar narrow band of infrared radiation of a wavelength that is not absorbed by the CO.sub.2. This second band of infrared radiation typically has a wavelength of approximately 3.7 microns and is adjacent to the band of absorbable radiation. The second detector also generates an electrical signal but one that is proportional to the magnitude of the radiation not absorbed by the CO.sub.2.
The output signals generated by the detectors are sent to the signal processor. The signals are ratioed to eliminate errors in the measured concentration of the CO.sub.2. These errors are attributable to such factors as foreign substances (e.g., condensation on the cuvette windows) and other instabilities in the infrared source and/or the detectors.
The gas analyzer disclosed in the '342 patent employs the above dual detector-filter pair but adds two additional components, a beam splitter and a lens configuration, to increase the sensitivity and accuracy of the detectors. A dichroic beam splitter is incorporated in the beam path ahead of the detectors. This beam splitter separates the two wavelengths to be measured out of the radiation spectrum and directs only the preselected wavelength to its corresponding detector. More specifically, the dichroic beam splitter is made of a material which transmits infrared radiation having a wavelength of 3.7 microns and permits the rays of this wavelength to impinge upon the "reference" detector. On the other hand, rays having a radiation wavelength of 4.3 microns are reflected and directed to the "measuring" detector. The added lens configuration consists of a lens positioned in front of the beam splitter which focuses the beam, after it passes through the sample cell, onto the detectors. Both of these additional features reduce the detector inaccuracy due to an obscured optical path caused by condensation and foreign particles on the cuvette windows.
One disadvantage of this lens configuration is the heightened possibility of dissymmetry between the two preselected wavelength beam paths caused by the cumulative effect of lens tolerance error. In other words, even a minor tolerance error produced by obstruction of the optical path is multiplied by some factor as the obscured beam is focused by each lens. Especially if the error is present in only one of the beam paths, the resulting dissymmetry between the two paths gives a less accurate CO.sub.2 concentration measurement.
Therefore, it is desirable to design a gas analyzer which accurately focuses the beams of selected wavelengths without increasing dissymmetry between the two selected beam paths. The present invention proposes to accomplish this by an improved lens-free design. In addition to aiding in the symmetry of the detector measurements, the elimination of the lenses makes for a less expensive and lighter weight unit.
Again referring to the prior art systems disclosed in U.S. Pat. Nos. 4,859,858, 4,859,859, and 4,194,720, because of the juxtaposition of the two detectors, the infrared radiation reaching both detectors will, for all practical purposes, be attenuated equally by condensation on the sample cell windows and contamination along the optical path between the infrared radiation emitter and the detectors. Also, it will be equally affected by thermal drift and variations in the ambient temperature. To help eliminate any error caused by condensation, contamination, thermal drift or ambient temperature variations, the signals generated by the detectors are ratioed.
The above prior art systems preferably employ lead selenide detectors for their high sensitivity and comparably low cost. However, lead selenide detectors are very temperature sensitive with temperature variation affecting the bulk resistivity and the sensitivity of the detector material. Errors that would be produced by variations in detector temperature cannot be eliminated by employing the ratioing technique discussed above. In fact, the signal from the infrared radiation detector can be lost, despite ratioing, if the detector temperature varies as little as 0.1.degree. C. Therefore, to maintain the detectors at a constant and precise temperature, a detector heater arrangement is provided in addition to employing the ratioing technique. This heating of the detectors also keeps unwanted condensation from forming on the optical components of the assembly in which the detectors are incorporated.
This prior art heating arrangement employs a strip heater of conventional construction and a thermistor for sensing the temperature in the detector unit. Both the strip heater and the thermistor are juxtaposed adjacent and in intimate heat transfer relationship to the detector and the housing or casing of the detector unit. The thermistor sends an analog signal to temperature control circuitry which converts that signal into digital form. This digital temperature signal is utilized in a feedback loop to control the duty cycle of the strip heater.
The disadvantage of this heating arrangement is that the sample cell windows can only by heated indirectly by means of heat transfer from the detector unit. Thus, condensation on the sample cell windows is not effectively evaporated.
In lieu of the above-described heating arrangement, other prior art arrangements employ a sample cell window heating system. For example, U.S. Pat. No. 5,092,342 discloses gas analyzer wherein electrical heating elements are applied directly to the surface of one or both of the sample cell holder windows on the respective sides facing away from the sample cell. Several electrically conductive band or track configurations are disclosed wherein the beam path is unobstructed. The heated windows are in a virtual touching contact engagement with the sample cell when the latter is seated in the gas analyzer bench so as to allow for a good heat transfer from the band to the sample cell windows. In this way, the condensation of water vapor which can occur on the inner surface of the sample cell windows is avoided.
A heat transfer relationship also exists between the detector assembly and the sample cell window closest to it. For this purpose, the block containing the detector is made of a thermally-conductive material. A temperature sensor embedded in the detector block or positioned between the block and the heated window is employed to control the temperature of the gas analyzer.
This heating arrangement is limited, however, in that the temperature of the detector block and the sample cell window(s) are dependent upon each other. In other words, there is only one source of heat, i.e, the window heaters, and one temperature sensor for both.
The prior art systems disclosed in U.S. Pat. Nos. 4,859,858, 4,859,859 and 4,194,720 each include a hand-held unit, remote from the gas analyzer bench, which contains all the electronic control, processing and power circuitry of the device. With this design, the electronic circuitry is housed remote from bench components. The remote unit holds a microcomputer, an analog-to-digital converter, signal processing circuitry and power supply circuitry. The microcomputer controls the operation of the infrared emitter, a heater which keeps the detectors at a constant, precise temperature, and the displays of a variety of information concerning the gas being measured. The analog-to-digital converter converts the signal emitted by the detectors to a ratioed signal indicative of the concentration of CO.sub.2 in the respiratory gas sample. The signal processing circuitry controls the operation of the gas analyzer and processes the detector-generated signals to display such medically-relevant information as minimum inspired CO.sub.2, respiration rate, and end tidal CO.sub.2, in addition to instantaneous concentration of CO.sub.2.
By locating the system's control circuitry remote from the gas analyzer bench, the above systems have the disadvantage of being required to use the same bench with the same hand-held electronics unit. This matching is necessary because each gas analyzer's infrared source, detector(s) and filter(s) have unique operating specifications and performance characteristics which are accommodated for by the companion electronics unit. The lack of interchangeability between the bench and electronics requires a user to use caution in pairing a bench with its corresponding electronic unit to avoid a calibration mismatch. Even if such a system provided interchangeable benches, information characterizing the individual gas analyzer's performance must be available to the processing means if the host monitor is to accurately interpret the signals from the detector(s). This would require the gas analyzer to be calibrated each time it is plugged into a corresponding host unit.
Furthermore, these systems are cumbersome due to the plurality of components, i.e., gas analyzer bench, electronics unit, and interconnecting cable. There is also the additional disadvantage of the distortion and noise attributable to the transmission of electronic signals via an external cable.
Therefore, it is desirable to design a gas analyzer system in which the individual gas analyzer benches are detachable and house their own electronics. Such a design will allow the benches to be interchanged with any host monitor and eliminate the risk of distortion and noise affects due to external transmission lines. It is also desirable that such interchangeable benches be capable of storing their respective calibration characteristics and of providing such information to the corresponding host unit.
The prior art systems disclosed in U.S. Pat. Nos. 4,859,858, 4,859,859 and 4,194,720 further disclose a manner of engaging the gas analyzer bench with the sample cell using detents and complementary recesses. A spherical detent and spring detent combination is disposed in a bore in the bench housing. The spherical detent can be trapped in one of four complementary recesses in the center section of the sample cell casing so that the bench may be coupled thereto in any one of several orientations. The purpose of this design is to facilitate engagement of the bench and cell by reducing the precision needed to assemble them.
A drawback of the prior art design is that the movement of the detents results in wear on the casing which may eventually lead to enlarged recesses making the fit between the bench and cell less secure. The likelihood of this is even greater when metal parts (e.g., spring detents) are used with a plastic casing. A loose fit could lead the optical beam path to drift resulting in less accurate CO.sub.2 measurements. It is therefore desirable to design a gas analyzer system having a bench-sample cell mating configuration that does not lead to component wear.
Many prior art devices, including the gas analyzer disclosed in 5,092,342, employ an incandescent lamp as the source of infrared radiation. An incandescent lamp is desirable as an infrared source in that it is relatively inexpensive and has a rapid decay time and low power requirements. However, a drawback of the incandescent lamp is that it is not very rugged and therefore susceptible to failure if dropped. It is therefore also desirable to design a gas analyzer system having a low cost infrared radiation source which is both rugged and has good operating characteristics.
It is therefore an object of the present invention to provide an apparatus and method for carbon dioxide measurement which improves upon the prior art.
It is also an object of the present invention to provide an apparatus for carbon dioxide measurement which employs a lens-free gas analyzer bench design so as to increase accuracy of the carbon dioxide measurement.
It is also an object of the present invention to provide an apparatus for carbon dioxide measurement having improved thermal stability.
Another object of the present invention is to provide an apparatus for carbon dioxide measurement having a dual heating control system for independently controlling the temperature of the detector assembly and sample cell windows.
Another object of the present invention is to provide an apparatus for carbon dioxide measurement which shuts off the heating control system if the system malfunctions.
It is also an object of the present invention to provide an apparatus for carbon dioxide measurement in which a gas analyzer bench is provided which is detachable and interchangeable with all compatible host units.
It is also a further object of the present invention to provide an apparatus for carbon dioxide measurement with interchangeable benches each having stored calibration information so as to eliminate the need to calibrate the bench each time it is plugged into a host unit.
It is also a further object of the present invention to provide an apparatus for carbon dioxide measurement in which the gas analyzer bench can be assembled with the sample cell in a manner that reduces wear on the bench housing.
It is still a further object of the present invention to provide an apparatus for carbon dioxide measurement which employs an inexpensive infrared radiation source which is resilient to physical shock.
It is still a further object of the present invention to provide an apparatus for carbon dioxide measurement having integrated circuitry capable of accepting an electrical signal proportional in magnitude to the intensity of the infrared radiation received by a detection unit within the sensor and supplying an output indicative of the CO.sub.2 concentration in a patient's respiratory gases continuously at the end tidal peak, during the inspired phase, and indicative of the patient's respiration rate.
A more general object of the present invention is to provide an apparatus for carbon dioxide measurement that is less expensive and lighter in weight than prior art devices.